GB934098A - Improved method and apparatus for locating imperfections in the protective covering of an underground metal pipe line - Google Patents

Abstract

934,098. Locating faults in cables and pipelines; voltage measurement; measuring bridges; fault testing. BRITISH INSULATED CALLENDER'S CONSTRUCTION CO. Ltd. April 28, 1960 [April 28, 1959], No. 14546/59. Class 37. A method of detecting perforations in the insulating outer covering of an underground cable or pipe comprises feeding a direct current, interrupted at a very low frequency, to the cable or pipe and traversing the surface of the ground along the route with a pair of probes connected to a voltmeter. In Fig. 5, a pulsating direct current is fed from source 40 to an underground pipe 42 having an insulating outer covering 43. A perforation such as 45 in the outer covering 43 will result in a potential gradient occurring along the surface of the ground. This is detected by a voltmeter 44 carried by a man, represented by resistance 46 walking along the route. The man 46 must not wear highly insulating footwear and holds one terminal of the voltmeter 44; the other terminal being carried by a second man representated at 47. The pulsation of the supply current may be coded and is of a sufficiently low frequency to cause a visible variation in the voltmeter reading. A null is detected when the two men are equidistant from the fault 45. Alternatively, the men 46, 47 may use extended probes contacting the surface of the ground. Fig. 1 (not shown) relates to a constant-current power supply unit, the output of which is pulsated by means of mechanically controlled contacts. Portable valve voltmeters.-Fig. 4 shows a valve voltmeter suitable for use at 44 and comprising a bridge circuit formed by a pair of valves 30 and the two halves of a potentiometer 31. The bridge is initially balanced by adjustment of potentiometer 31 so as to give zero indication on the centre-zero microammeter 39. The bridge becomes unbalanced by a D.C. potential present between the manually-held probes 25 so that the deflection of instrument 39 is dependent on the value of the applied potential. The potentiometer 31 may be adjusted so that the pointer of instrument 39 remains on the scale despite the presence of extraneous unmodulated potentials between the probes 25. The instrument sensitivity may be changed by one operator holding part 27 of his probe so that a series resistor 29 is incorporated.